Optical sensors find a variety of uses, including satellite attitude sensors, video cameras and security systems. Many of these applications, such as a vehicle viewing system, are required to operate over an extensive range of conditions. For example, a wide intra-scene brightness range allows for viewing dimly lit night scenes even in the presence of glare from headlamps. A wide inter-scene brightness range allows for viewing scenes illuminated by bright sunlight as well as moonlight. Still further, frame rates must allow for displayed scenes to appear real-time.
Charge-coupled devices (CCDs) have often been the technology of choice. A CCD optical sensor operates as an analog shift register, passing charge developed in proportion to light incident on a pixel across adjacent pixels until the charge reaches an end pixel where it is processed. However cost, read-out rate limitations, requirements for high and multiple voltage levels, and support electronics integration incompatibilities have prohibited large-scale adaptation of CCDs into certain applications such as vehicle viewing systems.
Unlike CCDs, active pixel sensors (APSs) utilize at least one active element within each pixel to accomplish amplification, pixel selection, charge storage or a similar benefit. As such, APS devices have many of the benefits of CCDs including high sensitivity, high signal fidelity and large array formats. Because APS cells are accessed in a row-wise manner, the problems arising from transferring charge across pixel cells, as is done in CCD sensors, are alleviated. Additional comparisons between APS cells and other devices are presented in, for example, "Active Pixel Sensors: Are CCD's Dinosaurs?" in Proceedings of SPIE: Charge-Coupled Devices and Solid State Optical Sensors III, Vol. 30, pp. 2-14 (1993) by E. R. Fossum, which is hereby incorporated by reference.
One form of APS utilizes a photodiode p-n junction and a source-follower buffer in each pixel. However, photodiode devices typically suffer from high kTC, 1/f and fixed pattern noise, thereby limiting dynamic range.
An alternative APS design uses a metal-on-silicon (MOS) photogate to accumulate charge proportional to light incident during an integration period. The charge can be shifted to a sensing region for readout. The sensing region can also be reset, allowing a reference output indicative of noise levels. The reference can be subtracted from the integrated light value to implement correlated double sampling.
A photogate device presents several benefits. A first benefit is that photogates have a very low noise level compared to other devices such as photodiodes. This results in the need for less integration time to achieve a desired light sensitivity. A second benefit is that the photogate APS is compatible with standard CMOS manufacturing methods. This allows an APS array together with control and processing circuitry to be built on the same integrated circuit chip.
In order to accommodate wide intra-scene and inter-scene brightness levels, an increased dynamic range is required. This can be accomplished by increasing the integration time used by each pixel cell in the optical sensor. Traditionally, integration time has meant a corresponding increase in frame time. Since frame time determines the rate at which the output image is updated, increasing frame time may result in output images that no longer appear real-time.